FIG. 23 shows a general configuration of a conventional high-frequency power supply system. The power supply system includes a high-frequency power source 51 to output high-frequency power, an impedance matching unit 53 for matching the input impedance of the high-frequency power source 51 with the load impedance, and a load L which is e.g. a plasma processing apparatus. The impedance matching unit 53 is connected with the high-frequency power source 51 via a transmission line 52 provided by a coaxial cable. The load L is connected with the impedance matching unit 53 via a load connecting section 54 consisting of e.g. copper plates and shielded to prevent electromagnetic leakage.
The high-frequency power source 51 is an apparatus for supplying the load L with high-frequency electric power. The high-frequency power source 51 includes unillustrated components such as a power amplification circuit and an oscillation circuit, and outputs high-frequency power of a predetermined electric power to the impedance matching unit 53 via the transmission line 52.
The impedance matching unit 53 matches the input impedance, i.e. an impedance viewed from the matching unit's inputting end toward the high-frequency power source 51, with the load impedance, i.e. an impedance viewed from the matching unit's inputting end toward the load L. The impedance matching unit 53 improves efficiency in supplying the output from the high-frequency power source 51 to the load L. The load L is an apparatus for processing works such as semiconductor wafers and liquid crystal substrates by means of etching, CVD, etc.
In the above-described high-frequency power supply system, the load L fluctuates while the load L is supplied with the high-frequency power from the high-frequency power source 51, resulting in unmatched impedance between the high-frequency power source 51 and the load L. In the high-frequency power supply system therefore, impedance matching between the high-frequency power source 51 and the load L is performed by automatically varying the impedance value of a variable impedance device (not illustrated) incorporated in the impedance matching unit 53 following the fluctuation of the load L.
Now, the high-frequency power supply system being as such, imagine that the load L is provided by a plasma processing apparatus and that a gas pressure change, an electric discharge temperature increase, etc. has triggered an arcing, an insulation breakdown or other abnormal state which causes abrupt impedance change of the load L. When this happens, the high-frequency power supply system or the impedance matching unit 53 can no longer catch up sufficiently in its impedance matching operation, and sometimes it becomes impossible to match the impedances. In such a case of unmatched impedance, a high-frequency power wave which is reflected back to the high-frequency power source 51 becomes very big, to damage the high-frequency power source 51. Meanwhile in the load L, the damaged component can be destroyed further by continued supply of the high frequency power.
In the high-frequency power supply system, unmatched impedance also occurs in the connection line between the high-frequency power source 51 and the load L if there is poor insulation, broken insulation, poor contact in a connector or other abnormal situation in the transmission line 52, the impedance matching unit 53, etc. Once this happens, a power wave which is reflected back to the high-frequency power source 51 becomes very big in the high-frequency power supply system, to damage the high-frequency power source 51 or exacerbate damage in the component where the abnormal situation originated.
Once such an anomaly occurs, it is desirable that some safety function takes place; for example the supply of high-frequency power may be stopped immediately. However, the conventional high-frequency power supply system is not designed to detect such an anomaly as described above or run a safety function.
In the field of high-frequency wave technology, parameters such as reflection coefficient and return loss are known as indicators of power supply efficiency to the load. By using these parameters it is possible to detect a reflected power wave which can damage the high-frequency power source 51. Based on this, it is possible as disclosed in e.g. JP-A 2000-299198 Gazette to monitor the status of the load by using the reflection coefficient Γ, to detect abnormalities by checking e.g. if the reflection coefficient is greater than a predetermined reference value, and to run a safety function.
However, a problem with this method is that the reflection coefficient should exceed the reference value in order for the anomaly to be recognized. Thus, there can be a situation where the reflection coefficient has changed and an anomaly has already occurred in the load yet the system has not determined that the situation is abnormal. In other words, this method does not have sufficient response for safety purposes. For example, see FIG. 24 and FIG. 25 which show time-course changes of a reflection coefficient. FIG. 24 shows a case where there is an abnormal, instantaneous surge in the reflection coefficient within a range not exceeding the reference value. FIG. 25 shows a case where there is a series of intermittent surges in the reflection coefficient within a range not exceeding the reference value. In such cases as shown in FIG. 24 and FIG. 25, the abnormalities are not detected even if they exist in the load as a result of change in the reflection coefficient. Further, the abnormalities will not be detected until they have grown to an extent that the reflection coefficient is greater than the reference value. Thus, the method is not adequate as a method for safety functions.